1. Field of the Invention
The present invention relates to an improved lithium secondary battery with a large capacity and superior charge - discharge cycle characteristics.
2. Description of the Prior Art
Lithium primary batteries using metallic lithium as the anode material, because of such advantages as high energy. density, light weight, small size and the ability to function well after long storage, have already been put into practical use in a variety of applications.
However, when lithium metal, which is so effective as an anode material for lithium primary batteries, is used as the anode material for a secondary battery, various new problems arise that were not foreseeable in the primary battery, and, consequently, it has not been possible to put lithium metal into practical use as the anode of a lithium secondary battery. The reason is that lithium secondary batteries having a lithium metal anode possess such serious flaws or defects as short charge - discharge cycle lifetimes and low charge - discharge efficiency. These flaws or defects are caused by the deterioration of the anode arising from an electrochemical reaction in which lithium metal is separated from solution by charging and discharging in the form of dendrites growing on the anode. In lithium secondary batteries possessing a lithium metal anode, it is not possible to avoid this kind of deterioration of the anode.
As stated above, when lithium metal is used as the anode of a secondary battery, the battery suffers from the defect of deterioration of the anode, and methods for using lithium - aluminum alloys as the anode material have been proposed to overcome this defect. Batteries prepared by these methods are at present widely used in such applications as memory backup for various small scale electronic devices. However, the danger originating in the use of lithium metal as a secondary battery anode is intrinsic to lithium metal, and there are serious problems involved in the application of such lithium aluminum alloy anode secondary batteries to such fields as power sources for portable electronic equipment that require a high capacity and in which nickel - cadmium batteries are presently used.
In addition, various secondary batteries having anodes made of carbon materials that occlude lithium ion have been proposed in place of batteries having anodes made of lithium metal.
For example, methods of using graphite as a lithium secondary battery anode material were disclosed in U.S. Pat. No. 4,304,825, Japanese Laid Open Patent Application No. 208079/1982, U.S. Pat. No. 4,423,125 and Japanese Laid Open Patent Application No. 102464/1983. However, since graphite crystallites are extensively developed, the destruction of the graphite crystalline structure occurs during the intercalation and deintercalation of lithium, and, as a result, the reversibility of intercalation-deintercalation is limited. Additionally, these batteries exhibit such defects as a large magnitude of self discharge and the decomposition of the electrolyte due to the high reactivity of lithium - graphite intercalation compounds. Consequently, such batteries are not applicable to practical use.
On the other hand, the use as a lithium secondary battery anode material of activated carbon, which is a carbon material possessing a high surface area, is disclosed in U.S. Pat. No. 4,497,883 et al. That patent discloses a device that makes use of electric double layer formation based on the high surface area of activated carbon However, the secondary batteries of that patent, which use the described activated carbon material as the anode, possess such defects as low charge - discharge efficiency and low battery voltage.
In order to solve these kinds of problems, the use as the anode material of carbon materials that differ both from graphite with its extensively developed crystallites and activated carbon with its high surface area has been proposed. In particular, the definition of carbon materials in terms of the pyrolysis temperatures used in their formation has been proposed, and a method for using pyrolyzed organic compounds obtained by heating organic compounds at temperatures of 1500.degree. C. or less as secondary battery anode materials is disclosed in Japanese Laid Open Patent Application No. 93176/1983 and U.S. Pat. No. 4,615,959. Moreover, the use of carbon fibers obtained by pyrolysis at around 2000.degree. C. as anode material is disclosed in Japanese Laid Open Patent Application No. 54181/1985, and, additionally, the use of carbon materials containing graphite structure and prepared by pyrolysis at temperatures ranging from 1000.degree. C. to 2500.degree. C. is disclosed in Japanese Laid Open Patent Application No. 221973/1985. On the other hand, the definition of carbon materials in terms of the values of their physical properties has also been proposed. For example, the use of a carbon material having a pseudographite structure in which the x-ray diffraction values for the interlayer spacing of carbon atoms (d002) and the size of the crystallites in the direction of the c axis (Lc002) are 3.39.ANG..ltoreq.d002.ltoreq.3.75 .ANG. and 8 .ANG..ltoreq.Lc002 .ltoreq.100 .ANG. respectively is disclosed in U.S. Pat. No. 4,702,977. Moreover, a method for using carbon materials that possess a surface area A (m.sup.2 /g) having a range of 0.1&lt;A&lt;100 and Lc002 and true density .rho. (g/cm.sup.3) values that simultaneously satisfy the conditions that 1.70&lt;P&lt;2.18 and 10&lt;Lc002&lt;120.rho.- 189 as anode materials is disclosed in U.S. Pat. No. 4,668,595. However, although the use of the carbon materials disclosed in the above patents and patent applications represents a significant improvement over the use of the above mentioned graphite and activated carbon, the battery capacity of batteries made with these carbon materials, particularly the capacity when the battery voltage is high, is still not adequate.
As the situation is understood, there has been a long-felt need by industry to provide anode materials that impart to the batteries made therefrom high capacity, specifically in excess of 500 mAh/g, and that exhibit superior characteristics in the constant potential region, that region in the anode charge - discharge curve (mAh/g versus potential) that is relatively flat and low in potential, and there have been substantial efforts by many people working in this field in recent years. Yet, despite all of this prior effort and the need for such an improved anode material, it has been left to the inventors of the present invention to provide a material that will satisfy this need.